Decanter centrifuges are sedimentation centrifuges used in clarification, dewatering and classification for the mixture of solids and liquid (slurry). Generally, the decanter centrifuge has a screw conveyor in a rotating bowl. The decanter centrifuge has a structure allowing solids, from a slurry introduced from a slurry feeding pipe inserted into the screw conveyor, to be sedimented on the inner surface of the rotating bowl due to centrifugal force. The solids are then scraped toward one end of the rotating bowl to be discharged by the screw conveyor rotating at a predetermined speed different from that of the rotating bowl. Simultaneously, separated liquid is discharged from another end of the rotating bowl by liquid pressure generated due to centrifugal force.
In conventional apparatus heretofore employed in decanter centrifuges of the above kind, dip weirs (baffles) are generally not provided. If the dip weirs are provided in such a device, efficient dewatering can not be attained. However, some decanter centrifuges provided with dip weirs have been proposed.
(a) Japanese Patent Application Laid-Open No. 59-169550 discloses a decanter centrifuge where a dip weir for cake-lay is provided and many blades are formed between the dip weir and a cake discharge port in order to obtain an orifice effect.
(b) Japanese Patent Application No. 62-43745 discloses a decanter centrifuge provided with a dip weir near a clarified liquid discharge port. Therefore, even if solid particles are attached on bubbles of separated water so as to float, the solid particles are prevented from being discharged due to the dip weir.
(c) Japanese Patent Application No. 1-19941 discloses a decanter centrifuge having a baffle disposed at the border between a conical portion and a straight shell. Then, the baffle can be rotatively adjusted to change a clearance between the baffle and a bowl so that efficient concentration can be performed.
(d) Japanese Utility Model Application Laid-Open No. 57-35849 discloses a decanter centrifuge having a baffle disposed at the border between a conical portion and a straight shell. Then, the conical portion and the straight shell can be separated clearly with the baffle so that efficient dewatering can be performed in the conical portion.
In each such conventional centrifuge, the baffle or the dip weir is provided in order to improve liquid-solids separating efficiency in the conical portion, while the straight shell has a mechanism for transporting the solids using conveyor means. Thus, in the conventional centrifuge, the straight shell does not have any mechanism for improving the efficiency for concentrating a feed solution or for efficiently dewatering a solids cake. Therefore, in the prior art, the water content in the cake depends on the degree of centrifugal force .produced in the straight shell, the length of the residence time of the feed solution in the straight shell, and the efficiency of dewatering of the feed solution in the conical portion.